1. Field of the Invention
The present invention relates to a backlight assembly, and more particularly, to a backlight assembly for a liquid crystal display device and a liquid crystal display device using the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for an arrangement in which light emitting diodes (LEDs) are used as light sources in a backlight assembly.
2. Discussion of the Related Art
In general, a display device portrays information as visual images. As communication technology has developed, a need for compact display devices has developed. Such display device can be classified as either a self-emitting type or a non-emitting type. A liquid crystal display (LCD) device is representative of a non-emitting type display device.
Since the LCD device is not a self-emitting type of display device, the LCD device requires an external light source. Accordingly, a backlight assembly, having a fluorescent lamp, is disposed on a rear surface of the LCD device. The backlight assembly radiates light into the liquid crystal panel of the LCD device, so that the LCD device can display visual images. The backlight assembly includes a light source, such as a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and/or LEDs. Among these light sources, LEDs have the characteristics of small size, low power consumption and stability. Accordingly, LEDs are becoming widely used as light sources in backlights for LCD devices.
FIG. 1 is an exploded perspective view of an LCD device using LEDs as a light source according to the related art. As shown in FIG. 1, the LCD device includes a liquid crystal panel 10, a backlight assembly 20, a main frame 40, a bottom frame 50 and a top frame 60. The liquid crystal panel 10 includes first and second substrates (not shown), facing each other with a liquid crystal layer (not shown) interposed between the first and second substrates, and a printed circuit board (PCB) 16 adjacent to the edge of the liquid crystal panel 10. The main frame 40 surrounds the sides of the liquid crystal panel 10 and the backlight assembly 20. The bottom frame 50 covers a rear surface of the backlight assembly 20 and can be attached to the main frame 40. The top frame 60 surrounds an edge of the liquid crystal panel 10 and can be attached to the main frame 40.
The backlight assembly 20 includes metal core printed circuit boards (MCPCBs) 22, LEDs 24, a reflecting sheet 26, a transparent window 30 and optical sheets 32. The MCPCBs 22 are disposed spaced apart from each other on an inner surface of the bottom frame 50. A plurality of LEDs 24 are disposed on each of the MCPCBs 22. The reflecting sheet 26 has through-holes 28 corresponding to the LEDs 24, respectively. The reflecting sheet 26 has a white or silver color. The reflecting sheet 26 covers the MCPCBs 22 and the inner surface of the bottom frame 50 such that the LEDs 24 protrude through the through-holes 28, respectively. The transparent window 30 has diverters 31 respectively corresponding to the LEDs 24. The optical sheets 32 can include a prism sheet, a diffusion sheet and other types of optical sheets. The transparent window 30 and the optical sheets 32 are sequentially disposed over the reflecting sheet 26.
FIG. 2 is a schematic view illustrating a sequential arrangement of LED strings of green, red, blue and green LEDs according to the related art. As shown in FIG. 2, a plurality of LED strings 24 are arranged on each of the MCPCBs 22. The MCPCBs 22 are arranged spaced apart from each other by a distance of about 60 mm on the inner surface of the bottom frame (50 of FIG. 1). Each of the LED strings 24 includes four LEDs in which the first and fourth LEDs are green LEDs 24a and 24d, the second LED is a red LED 24b and the third LED is a blue LED 24c. The plurality of LED strings 24 on each of the MCPCBs 22 emit light into the liquid crystal panel (10 of FIG. 1) and have a length l1 of about 40 mm. However, when the four LEDs 24a, 24b, 24c and 24b having green, red, blue and green colors, respectively, are used as an LED string in a backlight assembly (20 of FIG. 1), the brightness of the red LED 24b having a red color is not sufficient for proper color rendering on the liquid crystal panel (10 of FIG. 1).
FIG. 3 is a schematic view illustrating a sequential arrangement of LED strings of red, green, blue, green and red LEDs according to the related art. As shown in FIG. 3, a plurality of LED strings 25 sequentially arranged on the MCPCBs 22 are used as a light source of the backlight assembly (20 of FIG. 1) to resolve the insufficient brightness of the red LED 25b. Each LED string has five LEDs in which the second and fourth LEDs are green LEDs 25a and 25d, the first and fifth LEDs are red LEDs 25b and 25e, and the third is a blue LED 25c. A plurality of LED strings 25 are arranged on each of the MCPCBs 22. The MCPCBs 22 are arranged spaced apart from each other on the inner surface of the bottom frame (50 of FIG. 1).
FIG. 4 is a schematic perspective view illustrating LED strings using the method of field sequential control according to the related art. As shown in FIG. 4, a white light is produced by strobing a plurality of LED strings 25 at the same time and mixing the red, green, blue, green and red colors. The plurality of LED strings 25 may be strobed at the same time using a field sequential control (FSC). When the FSC is used, moving images are displayed on the liquid crystal panel (10 of FIG. 1) without image blur.
When the plurality of LED strings 24 or 25 having four or five LEDs are arranged on the MCPCB, the distance between adjacent LEDs on the MCPCB is close. The LEDs have an elliptical shape so as to mix and produce white light. The LCD device according to the related art has problems. For example, the close spacing concentrates heat buildup amongst the LEDs. Heat emitted from each of the LEDs decreases the life span of the LEDs if the heat is not dissipated away from the LEDs. Thus, an LCD device according to the related art using LEDs as a light source can have a short life span. Each of the LED strings 24 or 25 on the MCPCBs 22 can not be sectionally strobed to increase contrast because a combination of LED strings are needed to create white light. Since sectional control of the LED strings on the MCPCBs is not possible, the LCD device using LEDs as a light source according to the related art may have a low contrast.